Electric power companies must take into account the demand for electric power in order to generate and supply stable electric power to consumers. For this purpose, electric power companies always make efforts to have power stations for satisfying the largest demand, so that electricity can be generated according to the demand. However, as shown by the power demand curve A in FIG. 1, there is a large difference between a demand for power at daytime and that at night time. Similar phenomena can be observed for different time periods such as weeks, months or seasons.
Therefore, if it is possible to store electric power with high efficiency, surplus power, corresponding to the portion shown by X in FIG. 1, can be stored at an off-peak time and supplied at a peak time so that the portion shown by Y in FIG. 1 may be made up for by such stored surplus power. In such a manner, the supply of power can be made to correspond to changes in demand and the electric companies need only generate an almost constant power quantity corresponding to the broken line Z in FIG. 1 at all times. If such capacity load leveling can be accomplished, it is possible to allow the electric utilities to maximize the use of the most efficient base load plants.
Therefore, various power storage methods have been proposed. For example, a pumped hydro storage power generation method has been already practically utilized. According to the pumped hydro storage power generation method, a power station is located at a considerable distance from the place where power is consumed. Consequently, this method has disadvantages such as transmission and transformation losses and limited locations for the stations in view of environmental conditions. In consequence, it is desired to develop new power storing techniques in place of the pumped hydro storage power generation method. Redox flow batteries are being developed as one of such techniques.
FIG. 2 is a schematic structural view of an example of an already proposed redox flow battery. This redox flow battery 1 comprises a cell 2, a positive electrolyte tank 3 and a negative electrolyte tank 4. Since the two tanks 3 and 4 are used, this battery is called a 2-tank type battery. The cell 2 is separated by a membrane formed by an ion exchange membrane 5 for example, so that a positive electrode cell 2a and a negative electrode cell 2b are formed by the separation. A positive electrode 6 is provided in the positive electrode cell 2a, while a negative electrode 7 is provided in the negative electrode cell 2b.
The positive electrode cell 2a and the positive electrolyte tank 3 are connected through first and second pipes 11 and 12. The negative electrode cell 2b and the negative electrolyte tank 4 are connected through third and fourth pipes 13 and 14. A positive electrolyte is introduced as a reaction solution into the positive electrolyte tank 3 and a negative electrolyte is introduced as a reaction solution into the negative electrolyte tank 4. A pump P2 as reaction solution supply means is provided in the first pipe 11, while a pump P1 is provided in the second pipe 13. The positive and negative electrolyte react in the positive and negative electrode cells 2a and 2b, respectively. The positive electrolyte after the reaction returns into the positive electrolyte tank 3 through the second pipe 12, while the negative electrolyte after the reaction returns into the negative electrolyte tank 4 through the fourth pipe 14.
In the redox flow battery shown in FIG. 2, a solution of ions such as iron ions having a variable valence is used as the positive electrolyte and a solution of ions such as chromium ions having a variable valence is used as the negative electrolyte.
If hydrochloric acid solution containing a positive reactant Fe.sup.3+ /Fe.sup.2+ is used as the positive electrolyte and hydrochloric acid solution containing a negative reactant Cr.sup.3+ /Cr.sup.3+ is used as the negative electrolyte, reactions at the positive electrode 6 and at the negative electrode 7 are as follows: ##STR1##
An electromotive force of about 1 volt is obtained by the electrochemical reactions represented by the above formulas.
However, in reality, the above mentioned electrochemical reactions do not proceed equally at the positive and negative electrodes 6 and 7 as described above. This phenomenon is considered to be caused by the side reactions described below.
First, hydrogen gas is generated at the negative electrode at the end of a charging period and, as a result, an absolute quantity of oxidation-reduction pairs (Cr.sup.3+ /Fe.sup.2+ or Cr.sup.2+ Fe.sup.3+) is decreased.
Secondly, Cr.sup.2+ ions are relatively unstable and are liable to be oxidized by oxygen in the air and thus they are easily changed to Cr.sup.3+ ions. In such a case, the absolute quantity of oxidation-reduction pairs caused by the battery reactions is also decreased.
If the above described side reactions occur and the absolute quantity of oxidation-reduction pairs is decreased, electric energy stored in the battery, that is, the battery capacity is decreased as a result of a repetition of charging and discharging operations. Further, the internal resistance of the battery is increased and the charge and discharge efficiency often deteriorates.
In order to solve the above described problems, an apparatus for regenerating electrolyte of a redox flow battery has been disclosed in Japanese Patent Laying-Open No. 304580/1988.
FIG. 3 is a schematic diagram showing a construction of the apparatus for regenerating electrolyte of a redox flow battery described in Japanese Patent Laying-Open No. 304580/1988.
Referring to FIG. 3, a positive electrolyte tank 3 of the redox flow battery 1 is connected with the electrolyte regenerating apparatus 16 of the redox flow battery 1. Since the components of the redox flow battery 1 are the same as shown in FIG. 2, the description is not repeated.
The electrolyte regenerating apparatus 16 has a positive electrolyte chamber 20 and a negative electrolyte chamber 22 separated by a membrane 18. The positive electrolyte chamber 20 has a positive electrode 24 and the negative electrolyte chamber 22 has a negative electrode 26. Voltage applying means (not shown) for applying a voltage to the positive and negative electrodes 24 and 26 is connected to the positive and negative electrodes 24 and 26.
A gas-liquid separator 28 and a hydrochloric acid solution tank 30 are connected to the positive electrolyte chamber 20. Hydrochloric acid solution is supplied from the hydrochloric acid solution tank 30 into the positive electrolyte chamber 20 by means of a pump 32. The gas-liquid separator 28 separates chlorine gas generated in the positive electrolyte chamber 20, from the hydrochloric acid solution. A chlorine gas absorbing device 34 for absorbing the separated chlorine gas is connected to the gas-liquid separator 28.
The negative electrolyte chamber 22 is connected to the positive electrolyte tank 3 of the redox flow battery 1 so that the positive electrolyte in the positive electrolyte tank 3 of the redox flow battery 1 is supplied into the negative electrolyte chamber 22 and is discharged from the negative electrolyte chamber 22 to the positive electrolyte tank 3.
The above described electrolyte regenerating apparatus operates as follows when using Fe .sup.3+ /Fe.sup.2+ ions as the positive reactant and Cr.sup.3+ /Cr.sup.2+ ions as the negative reactant.
While charging and discharging operations are repeated in the redox flow battery 1, the amount of Fe.sup.3+ ions (or Cr.sup.3+ ions) of the oxidation-reduction pairs becomes excessive causing a deterioration of the electrolyte as described above. In this electrolyte regenerating apparatus, the excessive Fe.sup.3+ ions are reduced by using the electrolyte regenerating apparatus 16 as described below. Accordingly, the Fe.sup.2+ ions are regenerated and a normal balance of the oxidation-reduction pairs is maintained.
More specifically, the positive electrolyte supplied from the positive electrolyte tank 3 of the redox flow battery 1 to the negative electrolyte chamber 22 of the electrolyte regenerating apparatus 16 reacts according to the below indicated formula (1) when a voltage is applied to the electrodes 24 and 26. EQU Fe.sup.3+ +e.sup.- .fwdarw.Fe.sup.2+ (1)
Hydrochloric acid supplied from the hydrochloric acid solution tank 30 to the positive electrolyte chamber 20, reacts according to the below indicated formulas (2) and (3). EQU Cl.sup.- .fwdarw.1/2Cl.sub.2 +e.sup.- (2) EQU 1/2H.sub.2 O 1/40.sub.2 +H.sup.+ +e.sup.- (3)
Consequently, in the electrolyte regenerating apparatus 16, Fe.sup.3+ ions are reduced to Fe.sup.2+ ions at the negative electrode 26 and chlorine gas as well as oxygen gas are generated at the positive electrode 24. This chlorine gas is separated from the hydrochloric acid solution by the gas-liquid separator 28 and it is absorbed by the chlorine gas absorbing device 34. When the positive electrolyte containing the Fe.sup.2+ ions reduced by the electrolyte regenerating apparatus 16 is returned to the positive electrolyte tank 3 of the redox flow battery 1, the quantity of the oxidation-reduction pairs in the redox flow battery 1 is restored to the initial value.
If the electrolyte regenerating described above is connected to a redox flow battery, the balance of the oxidation-reduction pairs of the electrolyte of the redox flow battery is maintained normal and the battery capacity is restored.
However, since hydrochloric acid solution is used as a solution for electrochemically regenerating the positive electrolyte of the redox flow battery 1, noxious chlorine gas is generated in the positive electrolyte chamber, causing a safety hazard. In a practical use, safety can be ensured if the gas absorbing device 34 is provided. However, in such a case, the apparatus has a large size and the handling thereof becomes complicated.
In addition, since chlorine gas is treated, the apparatus needs to be made of a corrosion-resistant material. Furthermore, in order to prevent leakage of chlorine gas, it is necessary to take effective measures for air tightness of the apparatus. As a result, the apparatus is very expensive.